Dose monitor for plasma doping system

ABSTRACT

Plasma doping apparatus includes a plasma doping chamber, a platen mounted in the plasma doping chamber for supporting a workpiece such as a semiconductor wafer, a source of ionizable gas coupled to the chamber, an anode spaced from the platen and a pulse source for applying voltage pulses between the platen and the anode. The voltage pulses produce a plasma having a plasma sheath in the vicinity of the workpiece. The voltage pulses accelerate positive ions across the plasma sheath toward the platen for implantation into the workpiece. The plasma doping apparatus includes at least one Faraday cup positioned adjacent to the platen for collecting a sample of the positive ions accelerated across the plasma sheath. The sample is representative of the dose of positive ions implanted into the workpiece. The Faraday cup may include a multi-aperture cover for reducing the risk of discharge within the interior chamber of the Faraday cup. The Faraday cup may be configured to produce a lateral electric field within the interior chamber for suppressing escape of electrons, thereby improving measurement accuracy.

[0001] CROSS-REFERENCE TO RELATED APPLICATION

[0002] This application is a continuation-in-part of pending applicationSer. No. 09/128,370 filed Aug. 3, 1998.

FIELD OF THE INVENTION

[0003] This invention relates to plasma doping systems used for ionimplantation of workpieces and, more particularly, to methods andapparatus for measuring the ion dose implanted into the workpiece inplasma doping systems.

BACKGROUND OF THE INVENTION

[0004] Ion implantation is a standard technique for introducingconductivity-altering impurities into semiconductor wafers. In aconventional ion implantation system, a desired impurity material isionized in an ion source, the ions are accelerated to form an ion beamof prescribed energy, and the ion beam is directed at the surface of thewafer. The energetic ions in the beam penetrate into the bulk of thesemiconductor material and are embedded into the crystalline lattice ofthe semiconductor material to form a region of desired conductivity.

[0005] In some applications, it is necessary to form shallow junctionsin a semiconductor wafer, where the impurity material is confined to aregion near the surface of the wafer. In these applications, the highenergy acceleration and the related beam forming hardware ofconventional ion implanters are unnecessary. Accordingly, it has beenproposed to use plasma doping systems for forming shallow junctions insemiconductor wafers. In a plasma doping system, a semiconductor waferis placed on a conductive platen which functions as a cathode. Anionizable gas containing the desired dopant material is introduced intothe chamber, and a high voltage pulse is applied between the platen andan anode or the chamber walls, causing formation of a plasma having aplasma sheath in the vicinity of the wafer. The applied voltage causesions in the plasma to cross the plasma sheath and to be implanted intothe wafer. The depth of implantation is related to the voltage appliedbetween the wafer and the anode. A plasma doping system is described inU.S. Pat. No. 5,354,381 issued Oct. 11, 1994 to Sheng.

[0006] In the plasma doping system described above, the high voltagepulse generates a plasma and accelerates positive ions from the plasmatoward the wafer. In other types of plasma systems, known as plasmaimmersion systems, a continuous RF voltage is applied between the platenand the anode, thus producing a continuous plasma. At intervals, a highvoltage pulse is applied between the platen and the anode, causingpositive ions in the plasma to be accelerated toward the wafer.

[0007] Exacting requirements are placed on semiconductor fabricationprocesses involving ion implantation with respect to the cumulative iondose implanted into the wafer and dose uniformity across the wafersurface. The implanted dose determines the electrical activity of theimplanted region, while dose uniformity is required to ensure that alldevices on the semiconductor wafer have operating characteristics withinspecified limits.

[0008] One prior art approach to dose measurement in plasma dopingsystems involves measurement of the current delivered to the plasma bythe high voltage pulses, as described in the aforementioned U.S. Pat.No. 5,354,381. However, this approach is subject to inaccuracies. Themeasured current includes electrons generated during ion implantationand excludes neutral molecules that are implanted into the workpiece,even though these neutral molecules contribute to the total dose.Furthermore, since the measured current passes through the wafer beingimplanted, it is dependent on the characteristics of the wafer, whichmay produce errors in the measured current. Those characteristicsinclude emissivity, local charging, gas emission from photoresist on thewafer, etc. Thus, different wafers give different measured currents forthe same ion dose. In addition, the measured current pulses includelarge capacitive or displacement current components which may introduceerrors in the measurement.

[0009] A technique for plasma doping dosimetry is described by E. Joneset al. in IEEE Transactions on Plasma Science, Vol. 25, No. 1, February.1997, pp. 42-52. Measurements of implanter current and implant voltageare used to determine an implant profile for a single implant pulse. Theimplant profile for a single pulse is used to project the final implantprofile and total implanted dose. This approach is also subject toinaccuracies, due in part to the fact that it depends on power supplyand gas control stability to ensure repeatability. Furthermore, theempirical approach is time consuming and expensive.

[0010] In conventional ion implantation systems which involve theapplication of a high energy beam to the wafer, cumulative ion dose istypically measured by a Faraday cup, or Faraday cage, positioned infront of the target wafer. The Faraday cage is typically a conductiveenclosure, often with the wafer positioned at the downstream end of theenclosure and constituting part of the Faraday system. The ion beampasses through the Faraday cage to the wafer and produces an electricalcurrent in the Faraday. The Faraday current is supplied to an electronicdose processor, which integrates the current with respect to time todetermine the total ion dosage. The dose processor may be part of afeedback loop that is used to control the ion implanter.

[0011] Various Faraday cage configurations for ion implanters have beendisclosed in the prior art. Faraday cages positioned in front ofsemiconductor wafers are disclosed in U.S. Pat. No. 4,135,097 issuedJan. 16, 1979 to Forneris et al; U.S. Pat. No. 4,433,247 issued Feb. 21,1984 to Turner; U.S. Pat. No. 4,421,988 issued Dec. 20, 1983 toRobertson et al; U.S. Pat. No. 4,463,255 issued Jul. 31, 1984 toRobertson et al; U.S. Pat. No. 4,361,762 issued Nov. 30, 1982 toDouglas; U.S. Pat. No. 4,786,814 issued Nov. 22, 1988 to Kolondra et al;and U.S. Pat. No. 4,595,837 issued Jun. 17, 1986 to Wu et al. Faradaycages positioned behind a rotating disk are disclosed in U.S. Pat. No.4,228,358 issued Oct. 14, 1980 to Ryding; U.S. Pat. No. 4,234,797 issuedNov. 18, 1980 to Ryding; and U.S. Pat. No. 4,587,433 issued May 6, 1986to Farley.

[0012] Dose and dose uniformity have also been measured in conventionalhigh energy ion implantation systems using a corner cup arrangement asdisclosed in U.S. Pat. No. 4,751,393 issued Jun. 14, 1988 to Corey, Jr.et al. A mask having a central opening is positioned in the path of theion beam. The beam is scanned over the area of the mask with the portionpassing through the central opening impinging on the wafer. SmallFaraday cups are located at the four corners of the mask and sense thebeam current at these locations.

[0013] Accordingly, there is a need for improved methods and apparatusfor measuring ion dose implanted into a workpiece in plasma dopingsystems.

SUMMARY OF THE INVENTION

[0014] According a first aspect of the invention, plasma dopingapparatus is provided. The plasma doping apparatus comprises a plasmadoping chamber, a platen mounted in the plasma doping chamber forsupporting a workpiece, typically a semiconductor wafer, wherein theplaten and the workpiece constitute a cathode, a source of ionizable gascoupled to the chamber, an anode spaced from the platen and a pulsesource for applying high voltage pulses between the cathode and theanode. The high voltage pulses produce a plasma having a plasma sheathin the vicinity of the workpiece. The plasma contains positive ions ofthe ionizable gas. The high voltage pulses accelerate the positive ionsacross the plasma sheath toward the platen for implantation into theworkpiece. The plasma doping apparatus further comprises one or moreFaraday cups positioned adjacent to the platen for collecting a sampleof the positive ions accelerated across the plasma sheath. The sample isrepresentative of the dose of positive ions implanted into theworkpiece. The Faraday cup has a cover with a plurality of apertures.The ions collected by the Faraday cup pass through the apertures into aninterior chamber of Faraday cup and are detected.

[0015] Preferably, each of the apertures has a width that is less thanthe thickness of the plasma sheath. The width of each aperture isselected to prevent formation of a discharge within the interior chamberof the Faraday cup.

[0016] In one embodiment, the cover comprises a multi-aperture plate. Inanother embodiment, the cover comprises a wire mesh. In a furtherembodiment, the cover of the Faraday cup may comprise a front conductorfacing the plasma, a back conductor facing the interior chamber of theFaraday and an insulator separating the front conductor and the backconductor. The back conductor may be biased to repel electrons.

[0017] According to a second aspect of the invention, plasma dopingapparatus is provided. The plasma doping apparatus comprises a plasmadoping chamber, a platen mounted in the plasma doping chamber forsupporting a workpiece, typically a semiconductor wafer, wherein theplaten and the workpiece constitute a cathode, a source of ionizable gascoupled to the chamber, an anode spaced from the platen and a pulsesource for applying voltage pulses between the cathode and the anode.The voltage pulses produce a plasma having a plasma sheath in thevicinity of the workpiece. The plasma contains positive ions of theionizable gas. The voltage pulses accelerate the positive ions acrossthe plasma sheath toward the platen for implantation into the workpiece.The plasma doping apparatus further comprises one or more Faraday cupspositioned adjacent to the platen for collecting a sample of thepositive ions accelerated across the plasma sheath. The sample isrepresentative of the dose of positive ions implanted into theworkpiece. The Faraday cup includes means for producing within aninterior chamber of the Faraday cup an electric field lateral to thedirection of ions entering the Faraday cup.

[0018] The means for producing electric fields may comprise an electrodelocated within the interior chamber of the Faraday cup and a supplyvoltage coupled to the electrode for biasing the electrode with respectto a wall of the Faraday cup. When the interior chamber of the Faradaycup is cylindrical, the electrode may comprise an axial conductorlocated within the interior chamber. When the Faraday cup has an annularconfiguration, the electrode may comprise an annular electrode locatedbetween inner and outer walls of the Faraday cup. In another embodiment,an annular Faraday cup may have electrically isolated inner and outerwalls. A voltage may be applied between the inner and outer walls forproducing within the interior chamber of the Faraday cup an electricfield lateral to the direction of movement of the ions entering theFaraday cup.

[0019] The apparatus may include a single Faraday cup or two or moreFaraday cups disposed around the platen. The plasma doping apparatus mayinclude a guard ring, and the Faraday cup may be embedded within theguard ring. The guard ring can be maintained at either the cathodepotential or at another potential which is selected to control theplasma uniformity.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] For a better understanding of the present invention, reference ismade to the accompanying drawings, which are incorporated herein byreference and in which:

[0021]FIG. 1 is a simplified schematic block diagram of a plasma dopingsystem incorporating Faraday cups;

[0022]FIG. 2 is a partial schematic cross-sectional view of the plasmadoping system of FIG. 1, showing the wafer and the Faraday cups;

[0023]FIG. 3 is a partial schematic cross-sectional view of a plasmadoping system incorporating an annular Faraday cup;

[0024]FIG. 4 is a schematic cross-sectional diagram of a Faraday cup,illustrating a relatively thick plasma sheath that does not produce adischarge in the interior chamber of the Faraday cup;

[0025]FIG. 5 is a schematic cross-sectional diagram of a Faraday cup,illustrating a relatively thin plasma sheath that produces a dischargewithin the interior chamber of the Faraday cup;

[0026]FIG. 6 is a schematic cross-sectional view of a Faraday cupincorporating a cover having a plurality of apertures;

[0027]FIG. 7 is a schematic top view of the Faraday cup cover shown inFIG. 5;

[0028]FIG. 8 is an enlarged partial cross-sectional view of the Faradaycup shown in FIG. 6;

[0029]FIG. 9 is a schematic cross-sectional view of a Faraday cup havinga cover with a plurality of apertures and an electrode for repellingelectrons;

[0030]FIG. 10 is a schematic cross-sectional view of a Faraday cuphaving an axial electrode for producing a lateral electric field withinthe interior chamber of the Faraday cup; and

[0031]FIG. 11 is a partial perspective view of an annular Faraday cuphaving an annular electrode for producing a lateral electric fieldwithin the interior chamber of the Faraday cup.

DETAILED DESCRIPTION

[0032] An example of plasma doping system incorporating one or moreFaraday cups is shown schematically in FIG. 1. A plasma doping chamber10 defines an enclosed volume 12. A platen 14 positioned within chamber10 provides a surface for holding a workpiece, such as a semiconductorwafer 20. The wafer 20 may, for example, be clamped at its periphery toa flat surface of platen 14. The platen 14 supports wafer 20 andprovides an electrical connection to wafer 20. In one embodiment, theplaten has an electrically-conductive surface for supporting wafer 20.In another embodiment, the platen includes conductive pins forelectrical connection to wafer 20.

[0033] An anode 24 is positioned within chamber 10 in spaced relation toplaten 14. Anode 24 may be movable in a direction, indicated by arrow26, perpendicular to platen 14. The anode 24 is typically connected toelectrically-conductive walls of chamber 10, both of which may beconnected to ground.

[0034] The wafer 20 and the anode 24 are connected to a high voltagepulse generator 30, so that wafer 20 functions as a cathode. The pulsegenerator 30 typically provides pulses in a range of about 100 to 5000volts, about 1 to 50 microseconds in duration and a pulse repetitionrate of about 100 Hz to 2 KHz. It will be understood that these pulseparameter values are given by way of example only and that other valuesmay be utilized within the scope of the invention.

[0035] The enclosed volume 12 of chamber 10 is coupled through acontrollable valve 32 to a vacuum pump 34. A gas source 36 is coupledthrough a mass flow controller 38 to chamber 10. A pressure sensor 44located within chamber 10 provides a signal indicative of chamberpressure to a controller 46. The controller 46 compares the sensedchamber pressure with a desired pressure input and provides a controlsignal to valve 32. The control signal controls valve 32 so as tominimize the difference between the chamber pressure and the desiredpressure. Vacuum pump 34, valve 32, pressure sensor 44 and controller 46constitute a closed loop pressure control system. The pressure istypically controlled in a range of about one millitorr to about 500millitorr, but is not limited to this range. Gas source 36 supplies anionizable gas containing a desired dopant for implantation into theworkpiece. Examples of ionizable gas include BF₃, N₂, Ar, PF₅ and B₂H₆.Mass flow controller 38 regulates the rate at which gas is supplied tochamber 10. The configuration shown in FIG. 1 provides a continuous flowof process gas at a constant gas flow rate and constant pressure. Thepressure and gas flow rate are preferably regulated to providerepeatable results.

[0036] In operation, wafer 20 is positioned on platen 14. Then thepressure control system, mass flow controller 38 and gas source 36produce the desired pressure and gas flow rate within chamber 10. By wayof example, the chamber 10 may operate with BF₃ gas at a pressure of 10millitorr. The pulse generator 30 applies a series of high voltagepulses to wafer 20, causing formation of a plasma 40 between wafer 20and anode 24. As known in the art, the plasma 40 contains positive ionsof the ionizable gas from gas source 36. The plasma 40 further includesa plasma sheath 42 in the vicinity of platen 14. The electric field thatis present between anode 24 and platen 14 during the high voltage pulseaccelerates positive ions from plasma 40 across plasma sheath 42 towardplaten 14. The accelerated ions are implanted into wafer 20 to formregions of impurity material. The pulse voltage is selected to implantthe positive ions to a desired depth in wafer 20. The number of pulsesand the pulse duration are selected to provide a desired dose ofimpurity material in wafer 20. The current per pulse is a function ofpulse voltage, gas pressure and species and any variable position of theelectrodes. For example, the cathode to anode spacing may be adjustedfor different voltages.

[0037] One or more Faraday cups are positioned adjacent to platen 14 formeasuring the ion dose implanted into wafer 20. In the embodiment ofFIGS. 1 and 2, Faraday cups 50, 52, 54 and 56 are equally spaced aroundthe periphery of wafer 20. Each Faraday cup comprises a conductiveenclosure having an entrance 60 facing plasma 40. Each Faraday cup ispreferably positioned as close as is practical to wafer 20 andintercepts a sample of the positive ions accelerated from plasma 40toward platen 14.

[0038] The Faraday cups are electrically connected to a dose processor70 or other dose monitoring circuit. As known in the art, positive ionsentering each Faraday cup through entrance 60 produce a current in theelectrical circuit connected to the Faraday cup. The electrical currentis indicative of the number of positive ions received per unit time, orion current. It is assumed that the ion currents received by Faradaycups 50, 52, 54 and 56 have a fixed relation to the number of ionsimplanted in wafer 20 per unit time. Depending on the uniformity ofplasma 40 and the uniformity of ion acceleration toward platen 14, theion current per unit area received by each Faraday cup may besubstantially equal to or a fixed fraction of the ion current per unitarea implanted in wafer 20. Since the electrical current output of eachof the Faraday cups is representative of the ion current implanted inwafer 20, the Faraday cups 50, 52, 54 and 56 provide a measurement ofthe ion dose implanted in wafer 20.

[0039] As described in U.S. Pat. No. 5,711,812 issued Jan. 27, 1998 toChapek et al, the plasma doping system may include a guard ring 66 thatsurrounds platen 14. The guard ring 66 is biased to insure a relativelyuniform distribution of implanted ions near the edge of wafer 20. TheFaraday cups 50, 52, 54 and 56 may be positioned within guard ring 66near the periphery of wafer 20 and platen 14.

[0040] It will be understood that a variety of different Faraday cupconfigurations may be utilized. A second embodiment is shown in FIG. 3.An annular Faraday cup 80 is positioned around wafer 20 and platen 14.The annular Faraday cup 80 has the advantage that localized variationsin ion current are averaged around the periphery of wafer 20. Faradaycup 80 may be positioned in an annular guard ring 82. In general, anyconfiguration of one or more Faraday cups may be utilized. The Faradaycups are preferably located as close as is practical to wafer 20 andplaten 14. However, the Faraday cups may have any positions relative towafer 20 that provide a measurement representative of ion currentimplanted into wafer 20.

[0041] As indicated above, an electrical signal representative of ioncurrent is supplied from the Faraday cup or cups to dose processor 70.In one embodiment, the electrical current from each Faraday cup issupplied directly to dose processor 70 located external to chamber 10.In another embodiment, a preprocessing circuit (not shown) may belocated in close proximity to platen 14 and may operate at the voltageof platen 14. The circuit preprocesses the outputs of the Faraday cupsand supplies a result to dose processor 70.

[0042] The total ion dose delivered to wafer 20 is the instantaneous ioncurrent integrated over the time of the implant. The dose processor 70typically includes a circuit for integrating the outputs of the Faradaycups. The integrator may utilize conventional integrator circuits,charge sensitive amplifiers, or any other suitable circuit forperforming the integration function. Where the system includes two ormore Faraday cups, the outputs may be averaged to determine total dose.Dose processor configurations are known in connection with conventionalhigh energy ion implanters.

[0043] Two or more Faraday cups may be utilized to obtain a measure ofdose uniformity. Dose uniformity is the uniformity of implanted ionsover the surface area of wafer 20. With reference to FIG. 2, when theimplanted ion dose in wafer 20 is uniform, Faraday cups 50, 52, 54 and56 receive equal ion currents. When the dose is not uniform, the Faradaycups receive different ion currents. Accordingly, the current outputs ofthe Faraday cups may be compared with each other or with a reference toobtain a measure of uniformity. Thus, for example, if one or more of theFaraday cups provide an ion current that is different from the others,non-uniform ion implantation is indicated. The indication of non-uniformimplantation may, for example, be used to control the process, such asby stopping or altering the ion implantation.

[0044] The Faraday cup or cups used in the plasma doping system may havea variety of different configurations. In a basic configurationillustrated in FIG. 1, entrance 60 of each Faraday cup may be coplanarwith the surface of wafer 20 facing plasma 40. Each Faraday cup may beat the same electrical potential as platen 14 so as to minimize anydisturbance to plasma 40 by the Faraday cups.

[0045] The Faraday cup collects incoming charges and provides anelectrical signal that represents the net arrival rate of the charges.Therefore, secondary electrons which escape from the Faraday cupintroduce errors in the measurement of the ion current. For Faraday cupsthat operate in high vacuum, the common practice is to apply an electricfield and/or a magnetic field at the entrance to the cup, so thatsecondary electrons emitted as a result of ion bombardment of the cupcannot escape from the Faraday cup. Therefore the output signal is anaccurate measurement of the incoming ion beam current. Another way tostop the escape of secondary electrons is to design the Faraday cupgeometry in such a way that the electrons collide with the Faraday cupwall many times, and the probability of escape is low. The sameprinciples apply to Faraday cups which operate at lower vacuum, butprecautions are needed to prevent additional ionization processes withinthe Faraday cup.

[0046] Plasma doping systems operate at lower vacuum (higher pressure)than conventional beamline ion implantation systems. The gas pressure inthe Faraday cup is the same as the pressure in the plasma chamber. As aplasma is formed in the chamber, the strongest electric field is withinthe plasma sheath. When an electric field exists in the vicinity of theFaraday cup, gas molecules near the cup opening may be ionized andenhance a local discharge. Such enhancement is related to the geometricshape and dimension of the cup opening. It is known in the art that acylindrical shape can enhance the local plasma density up to two ordersof magnitude due to the hollow cathode effect. The enhanced localdischarge near the cup opening can extend into the cup to fill theentire interior chamber of the cup. Discharge within the Faraday cupdisturbs the surrounding plasma, reducing the uniformity of ionimplantation in the wafer. It also makes the Faraday cup output signaluseless, as it is no longer an indication of incident ion current.

[0047] The onset of discharge in the Faraday cup is a function ofFaraday cup geometry, material and surface condition, gas type andpressure, voltage applied to the plasma and current drawn from theplasma. A key factor in the onset of the discharge is the ratio ofFaraday cup opening size to plasma sheath thickness in a directionnormal to the platen. If this ratio is larger than one, the plasmasheath edge may be deformed by the opening, and such a deformationchanges the local electric field distribution. The change of electricfield distribution accelerates the deformation process and eventuallyleads to a discharge within the Faraday cup. If this ratio is less thanone, any deformation of the plasma sheath edge is minimal, and normaloperation of the Faraday cup as well as uniform ion implantation can bemaintained.

[0048] A Faraday cup 100 having an interior chamber 102 and an opening104 is shown in FIG. 4. A relatively thick plasma sheath 110 is formedadjacent to opening 104.

[0049] Equipotential lines are shown in FIG. 4 to represent the electricfield distribution near opening 104. The plasma sheath 110 is minimallydisturbed by opening 104, and no enhanced local discharge is formed nearopening 104 and in the interior chamber 102.

[0050]FIG. 5 shows Faraday cup 100 for the case of a relatively thinplasma sheath 120. The plasma sheath 120 is significantly disturbed byopening 104, causing a discharge 122 to form in interior chamber 102. Asnoted above, the discharge reduces the uniformity of ion implantationand renders the Faraday cup useless for ion current measurement.

[0051] Plasma sheath thickness is a function of plasma density and thevoltage applied to the plasma. Higher density and lower voltage producea thinner plasma sheath. Plasma doping requires a plasma density that issufficiently high to complete ion implantation within a short time. Highplasma density is usually achieved by increasing the gas pressure at lowimplantation voltages. This makes the plasma sheath thinner andincreases the risk of Faraday cup discharge. The effective range forusing the Faraday cup as a dose monitor is therefore limited due to thisdischarge problem.

[0052] A Faraday cup 200 having a configuration which reduces the riskof discharge within the Faraday cup is shown in FIGS. 6-8. Like elementsin FIGS. 6-8 have the same reference numerals. Faraday cup 200 includesa sidewall 210 a bottom wall 212 which define an interior chamber 216.Sidewall 210 may be cylindrical. An end of Faraday cup 200 facing plasmasheath 220 is provided with a cover 222 having multiple apertures 224.In the example of FIG. 6, cover 222 is a conductive multi-apertureplate. Ions enter the interior chamber 216 of Faraday cup throughapertures 224 and are detected. The purpose of the multi-aperture cover222 is to reduce the ratio of the aperture width W (see FIG. 8) to theplasma sheath thickness T, preferably to a value less than one, andthereby reduce the risk of discharge in the interior chamber 216 of theFaraday cup. The cover 222 is electrically isolated from sidewall 210 byan insulator 226.

[0053] The cover 222 may be configured such that the total opening areaof apertures 224 is the same or nearly the same as the area of a singlelarge aperture, so that the incident current remains the same or nearlythe same. The electric field in front of the multi-aperture cover 222 ismuch more flat than the electric field in front of a single largeraperture. The general rule is that, at a distance away from themulti-aperture cover which is greater than the width of each aperture,the electric field is substantially the same with or without theapertures. For plasma doping processes which produce a thin plasmasheath, the sheath thickness is still greater than the width of eachaperture. Electrons and ions at the plasma edge are not influenced bythese apertures, and the plasma is not disturbed. This eliminates thedischarge problem within the Faraday cup and extends the operationalrange of the Faraday cup to higher pressures and lower voltages.

[0054] The sidewall 210 and bottom wall 212 of Faraday cup 200 may beconnected through a current sensing device 230 to pulse source 30. Cover222 may be connected to pulse source 30 but is not connected throughcurrent sensing device 230. Preferably, pulse source 30 supplies thesame pulse to platen 14 (FIG. 1) and to Faraday cup 200, including cover222, so that a uniform electric field is seen by the plasma. The pulsevoltage corresponds to the desired implant energy and is typically in arange of about 100 volts to 5000 volts. The output of current sensingdevice 230 is an electrical signal that is representative of the numberof ions entering interior chamber 216 of Faraday cup 200, but excludesions that are incident on cover 222. Current sensing device 230, forexample, may be a Pearson coil or a battery operated circuit whichfloats at the same potential as the pulse source 30.

[0055] An enlarged partial cross-sectional view of Faraday cup 200 isshown in FIG. 8. The width W of each aperture 224 in plate 222 ispreferably less than the thickness T of plasma sheath 220 underoperating conditions of the plasma doping system which produce a thinplasma sheath. The apertures 224 may be circular, elongated or otherwiseshaped such that at least one dimension is less than the thickness ofthe plasma sheath. A spacing S between apertures 224 is preferablyminimized to provide a large total area of apertures 224. The spacing Sbetween apertures 224 may be on the order of the width W of apertures224 or less. A thickness t of cover 222 is preferably minimized to avalue required for structural integrity in order to reduce the number ofions intercepted by a wall 236 of aperture 224. A depth D of Faraday cup200, that is the spacing between multi-aperture cover 222 and bottomwall 212, is preferably long enough that escape of secondary electronsis minimized.

[0056] The cover shown in FIGS. 6-8 and described above is implementedas a multi-aperture plate. It will be understood that different coverconfigurations may be utilized within the scope of the invention. Anycover having apertures of suitable size to prevent discharge within theFaraday cup may be utilized. The cover may be fabricated of the samematerial as the workpiece to limit contamination of the workpiece. Thus,for example, the cover may be fabricated of silicon where silicon wafersare being implanted. In another embodiment, a wire mesh or screen may beutilized as a cover. The spacing between wires in the wire mesh ispreferably selected to provide apertures having widths that are lessthan the thickness of the plasma sheath.

[0057] A further embodiment of the multi-aperture cover is shownschematically in FIG. 9. Like elements in FIGS. 6-9 have the samereference numerals. A multi-aperture cover 240 includes a frontelectrode 242 and a back electrode 244 separated by an insulator 246.Apertures 224 are formed through front electrode 242, insulator 246 andback electrode 244. Front electrode 242 may be connected to pulse source30 (FIG. 6) to ensure that the plasma sees a uniform electric field.Back electrode 244, which faces the interior chamber 216 of the Faradaycup, may be biased to repel electrons. For example, back electrode 244may be biased negatively with respect to bottom wall 212. Thus, backelectrode 244 inhibits escape of secondary electrons from interiorchamber 216 through apertures 224.

[0058] The Faraday cup configuration described above works well for lowincident ion densities. The following feature of the invention dealswith the case of high ion densities. Ions generated in the plasma enterthe Faraday cup through its front opening, forming a positive chargecolumn within the cup. The charge density within the column is given bythe ion flux divided by ion velocity. When the ion density is highenough, the electric field created by these ions becomes significant.This effect, known as the space charge effect, must be taken intoconsideration when the ion density is above 10⁸ ions per cubiccentimeter. For plasma doping processes, the ion density entering theFaraday cup can be one or two orders of magnitude higher than 10⁸ ionsper cubic centimeter, depending on implantation parameters. The spacecharge effect causes ions to repel each other, so the cross section ofthe ion beam increases. When electrons are present, the positive chargecolumn functions as an electron trap. In other words, the positivecharge column forms a potential well, and electrons failing into thewell cannot escape unless they receive sufficient energy from externalsources. A few electrons may be bounced out of the trap due tocollision, but most electrons move along the column, in a directionopposite the ions, and may leave the Faraday cup. Electrons leaving theFaraday cup introduce errors in the output signal. Therefore it isimportant to separate the electrons and the positive charge column andto retain the electrons inside the cup in order to ensure an accurateion current measurement.

[0059] A Faraday cup configuration having reduced probability ofelectrons leaving the cup is shown in FIG. 10. A Faraday cup 300includes a sidewall 302, a bottom wall 304, a multi-aperture cover 306and an electrode 310. Sidewall 302, bottom wall 304 and cover 306 definean interior chamber 308 of Faraday cup 300. Multi-aperture cover 306 mayinclude a front electrode 312 and a back electrode 314 separated by aninsulator 316. Apertures 318 are formed through front electrode 312,insulator 316 and back electrode 314. In the embodiment of FIG. 10,sidewall 302 is cylindrical, and electrode 310 comprises a pin or rodlocated on the axis of the cylindrical sidewall. Electrode 310 iselectrically connected to bottom wall 304, and sidewall 302 iselectrically connected to the back electrode 314 of multi-aperture cover306. Sidewall 302 is electrically isolated from bottom wall by aninsulator 319. Electrode 310 is biased relative to sidewall 302 so as toproduce within interior chamber 308 an electric field 320 that islateral to the direction of ions entering Faraday cup 300. Thus, bottomwall 304 and center electrode 310 may be connected through a currentsensing device 322 to pulse source 30, front electrode 312 of cover 306may be connected to pulse source 30, and sidewall 302 and back electrode314 of cover 306 may be connected to a pulse source 324 through acurrent sensing device 326. The combined signal from current sensingdevices 322 and 326 provides an accurate measurement of the ion current.Pulse sources 30 and 324 are synchronized and produce pulses which biassidewall 302 positively relative to center electrode 310. In thecylindrical configuration of FIG. 10, the electric field 320 is radialand causes electrons to be pushed away from the positive charge columnof the ion beam.

[0060] The potential well created by the positive charge column isproportional to the charge density and cross sectional area of thecolumn. The potential difference from the center to the edge of a 1millimeter diameter charge column of density 3×10⁹ ions per cubiccentimeter is about 3.4 volts. The electric field strength at the columnedge is approximately 136 volts per centimeter. To push the electronsout of the potential well, a horizontal field that is greater than theelectric field created by the positive charge column is required. Thishorizontal field is introduced by applying a voltage between the centerelectrode 310 and the sidewall 302 of the Faraday cup. The electricfield is strong enough to push electrons to the sidewall 302, which actsas an electron collector, before the electrons leave the Faraday cup.The center electrode 310 and the bottom wall 304 act as an ioncollector. The net current (current from the ion collector minus thecurrent from the electron collector) gives an accurate measurement ofincident ion current. For a Faraday cup of one inch diameter, therequired voltage is approximately 200 volts. It is important to keep thevoltage low enough to avoid gas breakdown within the Faraday cup. ForBF₃ gas commonly used in plasma doping, the minimum breakdown voltage isabout 500 volts. By way of example only, pulses having amplitudes of −1kilovolt may be applied to electrode 310 and bottom wall 304, andsimultaneous pulses having amplitudes of −800 volts may be applied tosidewall 302 and the back electrode 314 of multi-aperture cover 306.

[0061] A Faraday cup configuration which produces a lateral electricfield in an annular Faraday cup is shown in FIG. 11. A Faraday cup 340includes an outer annular wall 342, an inner annular wall 344, a bottomannular wall 346 and a cover 354. An electrode 350 is disposed betweenouter wall 342 and inner wall 344. Electrode 350 is biased to producewithin interior chamber 348 of Faraday cup 340 an electric field 352lateral to the direction of ions entering the Faraday cup. Electricfield 352 is radial in the annular Faraday cup and pushes electrons awayfrom the positive charge column produced by the ions entering theFaraday cup. Electrode 350 has an annular configuration and iselectrically connected to bottom wall 346. The voltage pulses applied toouter wall 342, inner wall 344 and electrode 350 are selected to producelateral electric fields within the interior chamber 348 of the Faradaycup to push electrons away from the positive charge column. Cover 354may be provided with multiple apertures 356, which may be arc-shapedslots. As described above, the width of each aperture is preferably lessthan the thickness of the plasma sheath in the plasma doping chamber.

[0062] In another embodiment, outer wall 342 is electrically isolatedfrom inner wall 344, and electrode 350 is not utilized. In thisembodiment, outer wall 342 is biased relative to inner wall 344 toproduce a lateral electric field in the interior chamber 348 of theFaraday cup. Thus, for example, the voltage pulses applied to outer wall342 and inner wall 344 may differ in pulse amplitude by a selectedvoltage, such as 200 volts, to provide the desired lateral electricfield.

[0063] As described above in connection with FIGS. 6-9, the Faraday cupmay include a multi-aperture cover for reducing the risk of dischargewithin the Faraday cup. As further described above in connection withFIGS. 10 and 11, the Faraday cup may be configured, such as byincorporating an electrode, to produce a lateral electric field forsuppressing the escape of electrons from the interior chamber of theFaraday cup. It will be understood that these features may be utilizedseparately or in combination within the scope of the present invention.For example, electrode 350 may be omitted from the annular Faraday cup340 with multi-aperture cover 356. Alternatively, Faraday cup 340 withelectrode 350 may utilize a cover with a single annular aperture.

[0064] While there have been shown and described what are at presentconsidered the preferred embodiments of the present invention, it willbe obvious to those skilled in the art that various changes andmodifications may be made therein without departing from the scope ofthe invention as defined by the appended claims.

1. An ion implantation system for causing ions to impact an implantationsurface comprising: a process chamber defining a chamber interior intowhich one or more workpieces can be inserted for ion treatment; anenergy source for setting up an ion plasma within the process chamber; asupport for positioning one or more workpieces within an interior regionof the process chamber so that an implantation surface of the one ormore workpieces is positioned within the ion plasma; a pulse generatorin electrical communication with the workpiece support for applyingelectrical pulses for attracting ions to the support; one or moredosimetry cups including an electrically biased ion collecting surfaceparallel to the implantation surface; and an implantation controller formonitoring signals from the one or more dosimetry cups to control ionimplantation of the one or more workpieces.
 2. The ion implantationsystem of claim 1 wherein the one or more dosimetry cup have generallycircular apertures.
 3. The ion implantation system of claim 1 furtherincluding multiple dosimetry cups with apertures forming a sector ringshape.
 4. The ion implantation system of claim 1 wherein signals fromthe dosimetry cup that are used by the controller are derived fromelectric current through the collecting surface of the dosimetry cup. 5.The ion implantation system of claim 1 wherein the parameter of the ionimplantation process controlled is the duration of the implantation ofthe workpiece.
 6. The ion implantation system of claim 41 wherein thedosimetry cups are imbedded in the workpiece support to eliminate straycapacitance to ground thereby avoiding displacement current errors.